When a communication is established using a multivalue digital modulation system such as a 4-value FSK (frequency shift keying) modulation and the like, a Euclid distance in a significant bit element takes values as largely as possible as compared with another bit element that is relatively not significant by temporarily applying binary (or a value lower than a normal value) modulation. This process enables a practically equivalent function of correcting an error in data to be transmitted.
In addition, to avoid the concentration of energy due to a modulation bias or to allow confidentiality for information, data on a transmission line is frequently scrambled. Thus, by scrambling data, a modulation signal can be dispersed like pseudo noise (PN) although, for instance, original information data indicates an identical value (for a specific example, data indicating all “0”). Conventionally when data is scrambled, an exclusive logical sum (ExOR) of target information data and a scramble pattern as a prepared bit string has been determined by a logical operation circuit and the like (refer to, for instance, “Narrow Band Digital Communication System (SCPC/FDMA)” by Association of Radio Industries and Businesses, Standard Specification ARIB STD-T61 version 1.0, vol. 2, May 27, 1999, p. 142-143). The scramble pattern is often generated as a PN code sequence and the like. It is commonly designed that various scramble patterns can be generated by setting an initial value to be provided for a generator of a PN code as a variable scramble code.
Described below is, for an example, in the data transmission system of a frame structure configured by an information data set as a synchronous word and a function channel, a procedure of scrambling a predetermined data string in one frame. The synchronous word is a timing signal for use in performing a decoding process, and an information data set is a set of data classified for each function such as audio data, data for communication control, etc.
In this case, after performing an error correction coding process individually on each function channel to be included in one frame, the function channels are combined together to assemble into one frame. A frame in a non-scrambled state is completed, then an exclusive logical sum of a predetermined data string and a scramble pattern is determined, thereby generating a transmitting data string. Thus, the generated transmitting data string is converted into symbol data, and then used as modulating data in modulating a carrier wave and the like, and transmitted to a transmission line. On a transmission data reception side, the transmitting data is demodulated, and then descrambled in the inverse procedure of the scrambling procedure, thereby an original information data set and the like can be reconstructed.
Thus, the scrambling process on a data string forming one frame is performed on each bit value immediately before performing a conversion into symbol data.
As mentioned above, to set a longer Euclid distance in the significant bit element than the distances of other bit elements, it is important to appropriately arrange each piece of data in the symbol data. Therefore, if the scrambling process is performed on each bit value before performing a conversion into symbol data as in the conventional technique, it is difficult to sufficiently set the Euclid distance for a significant bit element.
Then, after scrambling a data string, a predetermined process is performed on a portion corresponding to a significant bit element, thereby possibly improving the error resistance of the portion. In this case, it is necessary to perform the process again on each function channel after once forming a frame. Therefore, there is the problem that the process is complicated when the position of a significant bit element is changed after the contents of the function channel are changed, for instance, such as when audio data is changed into data for communication control.